And that is what we need to beware of. The tendency comes in so many forms, a pride in our right doctrine as opposed to the fellows down the road who don't have it, a pride in our standing apart from the ecumenical movement, even a pride in our separation from those Churches that do not separate from error.
Do not misunderstand me, doctrine is important - but if we know the truth, then it is because we have been led to see it by God, it is not because we are smarter than others. Indeed, there are many really smart people who are atheists! You have nothing that you did not receive, and therefore you have nothing to boast about.You did not open your own eyes, God opened them for you.
Not having fellowship with the unfruitful works of darkness is important. There are many churches today that are not actually churches at all because Christ is not preached there - and there, but for the grace of God, we go. Again, it is Christ who keeps his people, we do not keep ourselves, and therefore the fact that we are not involved in such things as the ecumenical movement is a reason to fall on our knees and thank God, humbly, that he has kept us from being deceived.
The Pharisees earned their name from their desire to stand apart from the world - the name means 'separated ones'. There is much that is commendable in that desire, but sadly it took the wrong form. In their zeal to be 'separate' they added to the Law of God.
One of the worst manifestations of legalism is when a Church adds to the law of God. There is a story told of John Wesley, how he met a man who had been influenced by Russian Orthodoxy. The man said to Wesley words to the effect, "Mr. Wesley, you are a godly man full of zeal, one thing thou lackest - grow a beard." For this man, only men with beards could be saved.
The Bible condemns all those who said that "unless you are circumcised according to the custom of Moses, you cannot be saved." Now, God actually commanded circumcision under the Law, and yet those who forced that upon the Churches are condemned. What shall we say then to those who impose a law that God has not spoken? The man Wesley encountered was a legalist of that type, a man who has an extra-Biblical law that he wants to impose on others.
And now let me be controversial: teetotalism is not commanded in the Bible. That means that to tell a person "unless you abstain from all alcoholic beverages you cannot be saved" is to fall under the condemnation of the legalist. Now, the Bible is quite clear that drunkenness is a sin, being intoxicated with alcohol is wrong. But alcohol itself is never forbidden by God. I recently had the misfortune to be on the receiving end of an hysterical tirade from a man on the internet who, because I would not agree with him that alcohol is in and of itself evil, called me a false shepherd. That is the leaven of the Pharisees.
Modern myths notwithstanding, Jesus made alcoholic wine at the wedding at Cana - it is clear from the passage that the wedding took place shortly before Passover, but the grape harvest in Israel does not begin until July, and so there was no fresh grape juice available. Grape juice naturally ferments when stored, and the technology to pasteurize it was not available in ancient Israel, thus they had been drinking alcoholic wine before it ran out, and what Jesus made was like that which they had been drinking already.
Before someone accuses me of saying that Jesus was encouraging drunkenness, I should add that a wedding feast would have a very large number of people - commonly whole villages took part - and that this is in the context of a culture with a strong taboo against drunkenness.
Now, I am not saying "unless you drink alcohol you cannot be saved", that would be silly. Rather I am saying let he who drinks in moderation do it to the glory of God, and let he who abstains do it to the glory of God. But let neither judge the other in respect of drink, knowing it is to God, and not to you, that the other answers. Do not bind the conscience of the moderate drinker or the total abstainer. Tell the drunkard that he is mocking God by his act, and tell the legalist that he is taking God's name in vain. Let every man be persuaded in his own mind, and then let him do what he will, within the limits that God really has prescribed.
Beware of legalism. It kills. I have used the absolute forbidding of alcohol as an example, but there are many others. We are not wiser than God, and to order abstinence where God has ordered restraint is sinful.